DESCRIPTION (Investigator's Abstract): An examination of synapse formation and rearrangement ("synapse elimination") occurring in neonatal rat and mouse muscles is proposed. The long-term goal is to understand the mechanisms leading to the formation of specific synaptic connections and determining that all but one of the early synaptic connections are to be lost. The project has five specific aims. (1) To identify a cell-surface antigen present on muscle fibers which is responsible for the generation of fiber-type selective synaptic connections.Monoclonal antibodies will be prepared to membranes of fetal/neonatal muscles and screened for qualitative or quantitative differences in their binding to membranes of different fiber types. These antibodies will be used to examine the expression of candidate molecules during synaptogenesis in fetal and adult muscles. (2) To investigate a molecularcomponent of the neuromuscular junction recognized by monoclonal antibody 3G2. This monoclonal recognizes a previously undescribed, subsarcolemmal component of the junction whose expression requires innervation. This component appears at the junction during the time in early postnataldevelopment when synapse elimination is generating singly innervated fibers. The component recognized by this antibody will be characterized molecularly and its expression in fibers undergoing synapse elimination examined in detail. In addition, experiments will be performed to determine whether the neural control over the expression of the molecule is mediated by nerve-induced muscle activity or by nerve-released trophic factors. (3) To examine the role of differential neural activity and synapse elimination. A portion of the innervation of a muscle will be stimulated in order to examine whether active axons have any competitive advantage over less active axons in the process of synapse elimination. (4) To determine how homogeneity of fiber types within motor units is generated following the conclusion of synapse elimination. Motor units at the conclusion of synapse elimination differ in their size and fiber-type homogeneity from those in the adult, implying further readjustments in muscle innervation. The composition and size of motor units during the period immediately following the end of synapse elimination will be determined in order to investigate thses rearrangements. (5) To describe how fiber types differentiate in the opossum and to determine the role of the nervous system in promoting this differentiation. Opossums are born extremely immature; their hindlimbs are little more than limb buds. The differentiation f fiber types in the opossum hindlimb will be examined using antibodies to myosin heavy chains, and ablation of the lumbar spinal cord willbe performed to investigate the role of innervation in this differentiation and in the generation of secondary muscle fibers.